The state of the art pertaining to polymerization processes for producing commodity thermoplastics such as polyvinyl chloride (PVC) has undergone continual advancement. Significant improvements in chemistry and process interactions have occurred for example with suspension processes in the last two decades. Advances have been directed toward improving polymer properties as well as the following resin characteristics: porosity, residual monomer desorption, polymerization and stripping cycle times, compounding parameters, control of primary particle agglomeration, bulk density, reduction of particle fines and reactor fouling. Improvements of these properties still derive substantial economic advantages required for continued competitiveness in this large volume, maturing polymer technology.
There are examples of varied approaches directed toward achieving some the aforementioned improvements in resin quality. U.S. Pat. No. 4,603,151 to Dinbergs discloses a colloidal dispersant system to produce porous, spherical agglomerates by the use of substantially unneutralized crosslinked interpolymers of one or more carboxylic acid monomers, for example, a crosslinked polyacrylic acid polymer. The crosslinking is instrumental in the avoidance of a highly agglomerated charge, thereby preserving the spherical shape of the resin particles. Two other dispersion stabilizing surfactants are used in conjunction with the preferred crosslinked polycarboxylic dispersant. One is a polyether nonionic type and the other a non-polyether containing type. The purpose of these two surfactants is to improve the porosity. The result of employing the three dispersants is a significantly higher proportion of particles having spherical shape while maintaining low incidences of glassy particles.
U.S. Pat. No. 4,458,057 to Basu discloses a method directed at producing spherical, porous PVC resin particles and at the same time eliminating reactor build-up. The resulting particles are non-agglomerated, spherical, highly porous and have the same size distribution as the liquid dispersion. Prior to polymerization, the dispersant employed is a crosslinked water swellable, but water insoluble interpolymer of one or more carboxylic acid monomers. Partial neutralization of the acid is required prior to onset of polymerization.
U.S. Pat. No. 4,579,923 discloses improved porous PVC particles by the use of dispersants derived from an adduct of hydroxypropylmethacrylate and propylene oxide. The acrylate adduct is a comonomer for the vinyl chloride polymerization and is believed to provide steric stabilization of the PVC primary particles and improved attachment via ethylenic unsaturation as compared with prior art secondary surfactants, e.g. low hydrolysis polyvinyl acetate or esters of polyols such as sorbitol monostearate.
A background discussion of relevant work can be found in the The Encyclopedia of PVC, Vol. 1, 2nd ed., edited by Nass and Heiberger, Marcel Dekkar Inc., N.Y., pp. 110-127. Generally, conventional dispersant systems for suspension PVC are considered to consist of a combination of a primary suspending agent which primarily controls the particle size through control of interparticle agglomeration and optionally a secondary suspending agent which is believed to control porosity by controlling intraparticle agglomeration of primary particles. The primary suspension agents are typically cellulose ethers such as hydroxypropylmethylcellulose or partially hydrolyzed polyvinyl alcohols. Some conventional secondary dispersants are described as having a hydrophile-lipophile balance (HLB) ranging generally from about 4 to 10, thus being relatively higher in monomer solubility.
Commonly used secondary dispersants include sodium lauryl sulfate (U.S. Pat. No. 3,042,665), sodium dioctyl sulfosuccinate (U.S. Pat. No. 2,985,638) and sorbitan esters (U.S. Pat. No. 4,000,355). Low molecular weight polyvinyl acetates of low degrees of hydrolysis have been suggested. The use of secondary dispersants in many instances create colloidal stability upsets resulting in a particularly troublesome problem of reactor buildup or fouling. Reactor buildup is a serious problem where resin particles remain attached to sidewalls, baffles and fixtures. These show up as dark, discolored or gelled contamination on subsequent reactor charges as the buildup sloughs off. The use of secondary dispersants also can result in reduced particle size, excessive particle fines, and foaming during monomer stripping. Foaming problems occur generally when venting the reactor to the monomer recovery system wherein entrained foam passes resin into the recovery system causing clogging of recovery piping and contamination of the recovered monomer. Venting must be slowed considerably when this occurs, as such, foaming has a serious impact on resin quality and reactor cycle time.
Due to the importance of achieving sufficient porosity and short plasticizer powder mix times and the dependence on employing secondary dispersants to achieve such, it would be advantageous to employ porosity enhancing secondary dispersants which do not destabilize the colloidal suspension. It would also be desirable to employ dispersants without a significant reduction in resin particle size. Furthermore, other desirable features would include fewer incidences of particle fines, reduced foaming during monomer stripping and reduced reactor fouling.